Can't delete OU from AD, IsCriticalSystemObject attribute TRUE - cannot change

Now I'm trying to delete the two OU's but they have the isCriticalSystemObject attribute set as TRUE.

This is because those are the default locations for new computer and user objects in an SBS domain. The fact that you've removed SBS doesn't change the fact that this was set automatically by the SBS install when the domain was created. If you want to delete those OU's then you'll need to change the default location for new computer and user objects.

http://www.expta.com/2009/03/changing-default-users-and-computers.html

You have to (re)set the default location for new user/computer objects aded to the domani to another container/ou.

Set the OU/CN fpr new computers

C:\> redircmp OU=NewComputerOU,DC=domain,dc=tld

Set the OU/CN fpr new users

C:\> redirusr OU=NeueBenutzerOU,DC=domain,dc=tld

After that change was replicated, the old OUs can be deleted.